PHOTOSENSORY TRANSDUCTION IN CILIATES - ROLE OF INTRACELLULAR PH AND COMPARISON BETWEEN STENTOR-COERULEUS AND BLEPHARISMA-JAPONICUM

To test the hypothesis that light signal transduction in the unicellul ar ciliates Stentor coeruleus and Blepharisma japonicum involves a cha nge in intracellular pH as an initial signal following photoexcitation , we studied the dependence of the photophobic responses of the cells to changes in extracellular pH and to reagents that specifically affec t intracellular pH. The extracellular pH can modify not only the intra cellular pH, but can even reverse the sign of the pH gradient across t he cell membrane. Thus, as predicted by the hypothesis, low extracellu lar pH reversibly inhibited the photophobic response of the ciliates. The intracellular pH-modulating reagents tested included ammonium chlo ride, a membrane-permeable weak acid that lowers the intracellular pH, and the protonophores carbonylcyanide m-chlorophenyl-hydrazone (CCCP) and carbonylcyanide p-(trifluoromethoxy)-phenyl-hydrazone (FCCP), whi ch collapse the pH gradient across the cell membrane. The low pH and p rotonophore treatments caused a gradual inhibition of the photophobic responses in both ciliates. The observed reduction of the responsivene ss of the cells to visible light can be attributed to the alteration o f the intracellular pH, which is suggested to play a specific role in the photosensory transduction in both Stentor coeruleus and Blepharism a japonicum.